The present invention relates to electrochemical cells having an electrolyte and at least one electrode. More particularly, the invention relates to electrodes and electrode coatings for electrochemical cells. The coatings allow for the diffusion of an electrochemically active species through the coating during electrochemical release of the active species from the electrode or deposition of the active species onto the electrode while providing a substantially impervious barrier to the electrolyte. The electrode body having one of the coatings described herein may be made with various active materials and used in cells over a wide range of operating temperatures to deliver better electrochemical performance compared to the prior art.
In an electrochemical cell, chemical energy is converted to electrical energy with a reduction in the free energy of the system. In the course of the cell reaction, negative charge leaves the anode, travels through the external driven circuit, and re-enters the cell at the cathode. Thus, the cathode is the positive electrode and the anode is the negative electrode. By virtue of the established electromotive series, it is possible to select suitable cathodes and anodes to obtain a desired theoretical voltage. The ideal cell would give the theoretical voltage under continued, constant load, and the loss of free energy would manifest itself entirely as electrical energy outside the cell. However, this ideal is never attained in practice, because the internal resistance of the cell is not zero and the reactions within the cell never occur in a completely reversible manner. Moreover, incompatibility of the electrodes with each other or with the electrolyte, polarization, and other well-known problems prevent performance at theoretical values.
Certain materials are known to be more suitable than others for electrode materials. In particular, lithium has several properties which prove advantageous when used as an anode. Lithium in equilibrium with lithium ions in various solvent systems affords very negative potentials, and thus it is readily possible to achieve cell voltages greater than three volts, using appropriate oxide cathodic reactants, such as MnO.sub.2. Because lithium has the lowest equivalent weight of any metal, the high cell voltages result in high energy per unit weight of cell. Lithium is reasonably stable in many nonaqueous electrolyte systems. This stability arises as a consequence of a chemical reaction between the lithium anode and the electrolyte, forming a passivating film at the interface which limits further reaction. Such films cause a number of problems which are discussed below.
Over the past decade, many battery companies and government laboratories have worked on the development of ambient temperature lithium batteries. Several primary lithium cells and at least one secondary lithium cell are now commercially available.
In the field of primary cells, the Li-LiAlCl.sub.4 /SOCl.sub.2 -C system has been extensively investigated. This system has received particular attention because of its relatively high open-circuit voltage of 3.6 volts and high energy density of 500 W-hr/kg. Such lithium primary systems demonstrate very high energy densities. For example, the Li/SOC.sub.12 system exceeds 16-20 Wh/in.sup.3, which represents an energy density ten times greater than the Lechanche primary type. In addition to the questionable safety of the cell, however, there are other problems which hinder the ability of these systems to achieve their great potential for commercial success.
Using lithium as an anode material poses significant problems. Although lithium has been used successfully in aqueous electrolytes for very high drain rate batteries in military application, more conventional applications require the use of aprotic solvents to achieve reasonable shelf life and coulombic efficiency at low discharge rates. Use of these solvents causes handling and safety problems for such cells.
Lithium cells using a nonaqueous electrolyte have ruptured during use even though the cells were built to contain the gas generated by the electrochemical reaction between these components. Excessive internal heating, sometimes associated with inadvertent short circuits or over-discharging, is one reported cause of such rupture.
High drain-rate lithium cells have encountered problems with thermal runaway initiated by the exothermic reaction between lithium and the electrolyte, particularly an aqueous electrolyte. Since lithium has a relatively low melting point and its reactivity greatly increases upon melting, high operating temperatures must be avoided.
Lithium cells with liquid cathodes based on sulfurcontaining electrolytes have a particular problem with gas generated from the degradation of the electrolyte. Since the cells are sealed to prevent leakage of the electrolyte, the potential for the cell to rupture is dramatically increased.
Lithium cells operating at ambient temperature using liquid cathodes based on organic or inorganic electrolytes tend to suffer from a shortcoming known as voltage delay. This term describes a temporary voltage depression on load when cells are tested after extended periods of storage, especially at high temperature. This phenomenon results from lithium directly contacting a soluble depolarizer to form a passivating film at the anode surface. The factors controlling this delay in activation are not well understood. The passivation film is at least partially responsible for the chronic low rate capability that nearly all lithium battery systems suffer from.
The voltage delay phenomenon also plagues other types of cells. For example, in a magnesium dry cell a passivation film forms on the magnesium anode to limit corrosion. The cell is then unable to deliver full operating voltage after it is placed under load.
Attempts to solve the voltage delay phenomenon have generally concentrated upon additives to the electrolyte. Although some additives have reduced the effect of the voltage delay phenomenon, the voltage and overall electrochemical performance of the cell is significantly decreased.
An inconvenient and expensive method of avoiding the voltage delay phenomenon is to place a small rechargeable cell, i.e., nickel cadmium cell, in parallel circuit with the passivated cell. The rechargeable cell provides the operating voltage until the passivated cell is capable of doing so.
Passivation films often form before the cell is assembled to limit the operating voltage of a cell unless current densities are used above a barrier value. For example, this problem is exhibited by titanium anodes used in electrolytic manganese dioxide processes. Sandblasting and chemical washes are treatments used in the attempt to remove the passivation film before the anode is used in the process.
Anode materials like lithium also are inherently rechargeable, i.e., lithium can be electrodeposited from lithium ion-containing electrolytes. One of the major problems limiting the successful development of rechargeable versions of lithium cells is the nature of the lithium deposit during the recharging of the cells. Past investigations indicate that lithium plating can occur in dendritic form, lowering the cell's utilization efficiency and ultimately shorting the cell.
Attempts to prevent deleterious dendrite formation include alloying the lithium with other metals like aluminum. Electrolyte additives also have been used to promote surface alloy formation. Although cycle life of the cell improves, the power density of the cell significantly decreases. Other attempts employ cell separators, such as permeable membranes, to act as physical barriers to dendritic growth. Although cell separators are initially effective, lithium dendrites can eventually penetrate the cell separators and establish transient or permanent electronic shorts.
In order to increase the voltage generated by devices utilizing lithium metal, coatings or layers of lithium compound compositions have been used on the lithium metal contained in these devices. For example, U.S. Pat. No. 3,528,856, granted to Ovshinsky, discloses a high temperature voltage and current generating device including a layer of lithium metal which is coated by various lithium compound compositions, i.e., lithium oxide, lithium nitride, etc. The lithium metal, as coated, is useful for generating voltage and/or electrical power in response to the application of heat. Open circuit voltages of the order of 1.5 to 2.5 volts were observed when the device was exposed to high temperature.
An example of a lithium-air device which generates voltage by utilizing lithium compounds as a coating is disclosed in U.S. Pat. No. 3,615,835, granted to Ovshinsky. Various lithium compounds were used as a solid coating over lithium metal. The device operated at room temperature and was activated by exposure to water moisture which penetrated the coating to contact the lithium metal layer.